Accessory Dwelling Unit (ADU) Permit in Longmont, Colorado
Required for construction of a new attached, detached, or converted ADU in Longmont. Governed by the City's Land Development Code (Sections 15.04.040 and 15.02.080.E) and the Development Services Center ADU Guide, updated December 2025 to reflect changes required by Colorado HB24-1152 (statewide ADU legislation). LPC and Public Works both participate in ADU plan review alongside Building and Planning.
Verified 2026-06-30 · Source
When you need this permit
- Applicant must provide proof of Longmont residency: valid Colorado driver's license or state ID, plus at least two of: vehicle registration, voter registration, tax return/financial documentation, or a utility bill
- Applicant must own a detached home in Longmont that was legally permitted at time of construction
- Allowed only in zone districts permitting ADUs: Rural Residential (R-RU), Residential-Single Family (R-SF), Residential-Mixed Neighborhood (R-MN), Residential-Multifamily (R-MF), Mixed Use-Downtown (MU-D), Nonresidential-Agricultural (N-AG) — per Table 15.04.020 of the Land Development Code
- HOA covenants cannot prohibit ADUs (now unenforceable); HOA plan review/approval is optional but recommended and should be submitted with the permit application if obtained
- One ADU allowed per lot; up to 800 sq ft regardless of principal dwelling size, OR up to 50% of the principal dwelling's finished above-ground floor area if larger than 800 sq ft
- Total floor area of all accessory structures (including the ADU) capped at 75% of the principal structure's floor area, with specific exemptions for attached/internal ADUs
- Detached ADU height cannot exceed the height of the main house
- 3-foot all-weather (concrete or similar) sidewalk connection required linking the ADU entrance to the adjacent public sidewalk; wayfinding signage required if the ADU entrance isn't visible from the public right-of-way
- Setbacks (new detached ADU): front — not closer than the principal dwelling; rear — 5 ft; side — 5 ft
- ADUs may NOT be sold separately from the principal dwelling, and may NOT be leased/rented for tenancies of less than 30 days (no short-term rentals)
- Mobile homes, RVs, and travel trailers cannot be used as an ADU
- St. Vrain Valley School District fees must be documented as paid (contact Ryan Kragerud, Kragerud_ryan@svvsd.org, 303-682-7317)
Required documents
- Req
Accessory Dwelling Unit Application (via ACA portal)
Distinct ADU application type listed under Applications for Permit
- Req
Development Services Center Accessory Dwelling Unit Guide (Updated December 2025)
City of Longmont's current ADU standards, process steps, and submittal checklists by ADU type (detached / interior-remodel-basement-finish / addition)
- Req
Engineered, Stamped Foundation Plan and Shear Wall Design
Required for detached ADUs and additions
- Req
Survey / Improvement Location Certificate (ILC)
Showing existing structures and all property lines
- Req
Site Sketch
Existing structures, proposed structures, drainage, grading, and all property lines
- Req
Energy Compliance Path / Manual J, Manual D, Manual S
Required for detached ADUs and additions
- Req
Soils Report
Required for detached ADUs and additions
- Opt
Asbestos Testing Report
Required for interior-remodel/basement-finish or addition ADUs if demolition totals greater than 32 sq ft
Fee schedule
| Fee type | Amount | Notes |
|---|---|---|
| Total City Fees (typical range, all-inclusive) | $5,000–$10,000 | City of Longmont Development Services Center ADU Guide (Updated December 2025): 'Fees paid to the city typically range from $5,000–$10,000 and include plan review and permit fees, City and County taxes, and a number of community investment fees. The costs associated with an ADU are generally 65% less than constructing a new single-family home.' — exact figure depends on project valuation and applicable impact fees per the Primary Fee Schedule |
| Building Permit Fee — valuation-based (same Primary Fee Schedule as new construction) | Varies by valuation — see Residential Building Permit fee tiers | FY2026 Fee Manual; ADUs use the same valuation-based Primary Fee Schedule as other residential construction, plus applicable impact fees (Transportation, Parks, Recreation, Storm Drainage, ECIF) at the same rates as new construction |
Review timeline
~0–0 business days
Typical estimate — confirm current times with the Longmont building department
Inspection process
- 1
Foundation
Footings and foundation before concrete pour — 30-inch frost depth minimum
- 2
Framing
Structural framing per engineered/stamped plans
- 3
MEP Rough-In
Mechanical, electrical, plumbing rough-ins before walls closed
- 4
Final
All work complete per approved plans
Tips
- Longmont's ADU Guide was updated in December 2025 to reflect changes required by Colorado's statewide ADU legislation (HB24-1152) — confirm you're using the current December 2025 edition, not an older version, since standards changed materially.
- Proof of Longmont residency is a distinctive, non-obvious requirement — gather your driver's license/state ID plus two secondary documents (vehicle registration, voter registration, tax return, or utility bill) before applying.
- LPC (Longmont Power & Communications) is one of the reviewing departments during ADU Plan Check, alongside Public Works, Building, Planning, and Emergency Services — budget for multi-department review time.
- ADUs cannot be short-term rented (minimum 30-day tenancy) and cannot be sold separately from the principal dwelling — this differs from some other Colorado jurisdictions.
- St. Vrain Valley School District fee documentation is a distinct prerequisite specific to ADUs built within that district's boundaries — contact the district directly to confirm the amount owed before submitting.
Frequently asked questions
- Can I rent out my ADU short-term (e.g., Airbnb) in Longmont?
- No. Per the City of Longmont's ADU Guide, accessory dwelling units may not be leased or rented for tenancies of less than 30 days — short-term rentals are not allowed.
- How big can an ADU be in Longmont?
- An ADU may be up to 800 square feet regardless of the principal dwelling's size, or larger if it does not exceed 50% of the finished, above-ground floor area of the principal dwelling. If located in an existing basement, this size restriction does not apply. Total accessory structure floor area (including the ADU) is capped at 75% of the principal structure's floor area, with some exemptions for attached/internal ADUs.
- Does LPC review ADU applications in Longmont?
- Yes. During the ADU Plan Check phase, Longmont Power & Communications (LPC) reviews submitted plans alongside Public Works, Building, Planning, and Emergency Services to ensure compliance with all applicable codes and utility infrastructure requirements.
Sources & verification
Verified against official sources. Last reviewed 2026-06-30.
- City of Longmont — Building Services (Building Inspection Division), Development Services Center — official building department
- Accessory Dwelling Units (ADUs) — City of Longmont
- Development Services Center Accessory Dwelling Unit Guide (Updated December 2025, PDF)
- City of Longmont FY2026 Planning & Development Services Fee Manual (PDF)
Fees, timelines, and adopted codes are researched from each jurisdiction's published records — see how we verify. Requirements change and vary by project, so always confirm the current details with the Longmont building department before you submit. PermitBase is an independent reference and is not affiliated with any government agency.
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